1. Field of the Invention
The invention relates generally to methods and apparatus for (a) measuring ions in a liquid; (b) protecting such apparatus from the effects of electrostatic discharge; and (c) fabricating such apparatus, including electrostatic discharge protection circuitry, on a silicon wafer.
More particularly, the invention relates to methods, apparatus and chip fabrication techniques which provide electrostatic discharge (ESD) protection to ion-sensitive field effect transistor (ISFET) based devices used to selectively measure ions in a liquid.
According to one aspect of the invention, an ESD protection circuit, made up of conventional protective elements, is integrated onto the same silicon chip on which the ISFET is formed, along with a novel interface. According to the invention, the novel interface is a structure, in contact with the liquid being measured, which does not open up paths for D.C. leakage currents between the ISFET and the liquid.
According to a preferred embodiment of the invention, a capacitor structure is used as the interface between the protection circuit and the liquid sample.
Further aspects of the invention are directed to methods per se for providing ESD protection for ISFET sensors utilizing the interface means (e.g, capacitor structure) referred to hereinabove, and processes for fabricating the novel interface on a silicon wafer.
2. Description of the Related Art
Methods and apparatus for measuring ions in a liquid using an ISFET are well known to those skilled in the art. For example, in U.S. Pat. Nos. 4,020,830 to Johnson, and in 4,851,104 to Connery, both incorporated herein by reference to illustrate the state of the art, propose the use of such apparatus. Such devices typically include a measuring circuit and an ISFET immersed in a liquid for selectively measuring ion activity therein.
The aforementioned devices have numerous applications, including applications in the medical and biomedical fields where it is well known to use different ISFETs for measuring different ion activities, such as, for example, pH, pK and pNa.
While semiconductor FET type structures are known to be ESD sensitive, it was, until recently, believed by many that the ISFET structure was largely insensitive to ESD effects because (1) the ISFET does not contain a metallized gate electrode, (which usually is directly involved in electrostatic damage), and (2) experience over time indicated that such devices could be handled by many people without any evidence of electrostatic damage. Testing of ISFET electrodes has, however, indicated that large device shifts can occur following an ESD event.
Those skilled in the art will recognize that although attempts have been made to address the problem of preventing ESD damage in an ISFET based sensors; the known approaches for solving this problem contain inherent limitations, particularly when fabricating the sensor on a silicon wafer.
Before describing these limitations, examples of state of the art apparatus for protecting ISFET based sensors from ESD events will be set forth for comparison with the type of ESD protection methods and apparatus contemplated by the invention.
It will be demonstrated herein that ESD damage can be prevented (in accordance with the teachings of the invention) by using protection circuitry (within an ISFET based probe assembly) that allows build up of charge in a test sample during an ESD event, while simultaneously transferring the charge to the ISFET's source, drain and substrate.
This approach to providing ESD protection minimizes the field developed across the transistor's insulator structure by rapidly equalizing charges on either side of the insulator during an ESD event.
An ESD damage protection circuit for performing the above function could use, for example, fast bi-directional zener diodes connected between the test liquid and the transistor's source, drain and substrate conductors, as will be described in detail hereinafter with reference to the drawing.
The electrical contact to the test liquid can be achieved using a counter electrode, such as the counter electrode is described in the aforementioned U.S. Pat. No. 4,851,104, previously incorporated herein by reference.
Another example of state of the art apparatus for selectively measuring ions in a liquid and protecting against ESD events, is set forth by Ligtenberg et al., in U.S. Pat. No. 4,589,970. The U.S. Pat. No. 4,589,970 is incorporated herein by reference for its description of an ESD protection circuit used in an ISFET based sensor.
The ESD protection circuit taught in the incorporated U.S. Pat. No. 4,589,970 comprises at least one electrode, connected via a low impedance contact to the liquid being sampled, coupled to the ISFET by a protective element having a low impedance for high voltages and a high resistance to low voltages.
The incorporated U.S. Pat. No. 4,589,970 states that uni-directional zener diodes, capacitors, mechanical switches and high threshold voltage MOSFETS can be used in place of or in parallel with the bi-directional zeners to protect against ESD events.
The use of discrete components mounted in the probe package to form the protection circuit and/or the integration of the circuit onto the ISFET's silicon substrate is proposed by the incorporated U.S. Pat. No. 4,589,970.
As for the limitations referred to hereinbefore, one road block to the integration of the protection circuit described in the incorporated U.S. Pat. No. 4,589,970 (and any similar circuitry) onto the ISFET's silicon substrate is the difficulty in creating a metal electrode that provides a reliable low-impedance contact to the liquid.
The incorporated U.S. Pat. No. 4,589,970 suggests the use of aluminum or polysilicon films to form the contact; however, both films are subject to chemical attack in many of the liquids that would be measured by the ISFET.
An alternative would be to deposit a film comprised of a noble metal such as gold or platinum for the contact. Unfortunately gold and platinum films usually require an intermediate layer using materials such as titanium or chromium to provide good adhesion to the substrate; thus, the chemical resistance of the electrode could be compromised by the addition of another film especially if pinholes occur in the noble metal film.
Another concern that exists when a low impedance metal contact is used between the test liquid (sample) and the protection circuit is that paths are opened up for D.C. leakage currents via the protection circuit between the sample and the ISFET source, drain and substrate.
Accordingly, it would be desirable if methods and apparatus were available which provide ESD protection to ISFET based devices used to selectively measure ions in a liquid, using (a) an ESD protection circuit made up of conventional protective elements integrated onto the same silicon chip on which the ISFET is formed; and (b) an interface means, in contact with the liquid being measured, which does not open up paths for D.C. leakage currents between the ISFET and the liquid.
Furthermore, it would be desirable to provide fabrication techniques which would allow the aforementioned protection circuit to be integrated onto the ISFET's silicon substrate in a manner that avoids the difficulty in creating a metal electrode to serve as a reliable low-impedance contact with the liquid.
Further yet, it would be desirable to provide apparatus which utilize contact films that are resistant to chemical attack in many of the liquids that would be measured by the ISFET, without having to resort to the use of noble metals and intermediate layers to provide good adhesion for the film to the substrate.